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Joe Lykken ASCAC Meeting April 17, 2018 The age of and quantum communications is getting closer

Even after allowing for media hype, very impressive progress in the last few years on building real quantum and performing long- range quantum communication

2 4/17/18 Lykken | Perspectives on QIS | ASCAC What is a quantum digital ?

• Information is stored and manipulated as quantum states “” • A single state is in general a of two distinct states, which we will denote as state |0> and state |1>:

• The two angles parameterize the surface of a sphere, called the • Classical computing only accesses the North and South poles of the Bloch sphere

3 4/17/18 Lykken | Perspectives on QIS | ASCAC Quantum circuits • Given some number of qubits in arbitrary states, I can measure each qubit state • The measurement projects the n-qubit to an n- classical binary number • We call this a projection to the “computational basis” • This is the simplest

• More generally we can perform a variety of unitary operations, called gates, on the qubits before making measurements • Gates operations can act on a single qubit, or on multiple qubits • Starting from a rather small menu of gates one can obtain a universal quantum computer

4 4/17/18 Lykken | Perspectives on QIS | ASCAC Can quantum circuits solve hard problems? • Here is a quantum circuit that maps a 3-qubit input state

|x1>|x2>|x3> to an output state |y1>|y2>|y3>, where

• This is a (discrete) Fourier transform • The quantum circuit uses two kinds of gate operations:

• A Hadamard gate that acts on a single qubit: • A controlled phase shift: a two qubit gate that applies a phase shift to the target qubit that depends on the state of the control qubit

5 4/17/18 Lykken | Perspectives on QIS | ASCAC Exponential speedup

• The discrete Fourier transform is an example of a calculation that a quantum computer can do exponentially faster than any classical computer: for n qubits we need ~ n2 gate operations, whereas a conventional Fast Fourier Transform requires ~ n2n operations • In 1994 showed that factorization of prime numbers can be done with a combination of quantum Fourier transforms and classical operations that run in polynomial time • Thus a quantum computer with a sufficiently large number of qubits and reliable single and 2-qubit gate operations can do at least one important calculation exponentially faster than a classical computer

6 4/17/18 Lykken | Perspectives on QIS | ASCAC Rapid progress in building quantum computers Moore’s Law is over (?) and has been replaced by Martinis’ Law (?): the number of qubits in high performance quantum computers will double every 18 months

Google 22-qubit quantum computer, 2017 72-qubit quantum computer, 2018

A performant 72-qubit quantum computer can achieve “” = the ability to do at least some (not necessarily useful) calculations that cannot be reproduced by the world’s largest classical

7 4/17/18 Lykken | Perspectives on QIS | ASCAC Analog computers

• Analog computers are in principle more powerful for certain applications than universal digital computers

• Classical analog computers were the dominant form of computing until fairly Antikythera mechanism, recently circa 100 BCE

Soap bubble

Tide calculator USA until 1965

8 4/17/18 Lykken | Perspectives on QIS | ASCAC 1981 “ isn’t classical, dammit, and if you want a simulation of nature, you’d better make it quantum mechanical”

• First person to propose the idea of quantum computing and explain it’s potential importance

• In addition to universal digital quantum computers, proposed quantum analog computers where the quantum behavior of the one system simulates the quantum behavior of some other system that you want to better understand

• In other words, use quantum hardware to simulate quantum problems

• This is very different from building a gate-based quantum computer to do prime factorization, which is a classical problem

9 4/17/18 Lykken | Perspectives on QIS | ASCAC Challenges of building quantum computers Need a qubit, i.e. a two-state quantum system, that you can manipulate and not confuse with other possible states of the system. You must be able to create states that are superpositions of your two basis states, and maintain the quantum long enough to perform a series of quantum manipulations

Examples of qubits: • polarization • in two time bins • , nuclear spin, atomic spin • and particular of an ion • Ground state and first excited state of a nonlinear oscillator, e.g. a superconducting Josephson Junction LC circuit

10 4/17/18 Lykken | Perspectives on QIS | ASCAC From qubits to quantum systems

For good qubits you need • Quantum coherence • Pairwise coupling of qubits (for 2-qubit gates) • Ability to create initial state, perform qubit gate operations, and measure final state • Low rate of errors; ability to detect errors and do error correction Then scale this all up to a large quantum system

Extra challenge: you cannot copy (no-cloning theorem)

11 4/17/18 Lykken | Perspectives on QIS | ASCAC Superconducting quantum computers

Lots of groups working with variations on superconducting Josephson Junction circuits, in some cases with very fast and very high fidelity (low error rate) gate operations

Similar performance with 2-qubit gates: 99.4%, 40 ns

John Martinis superconducting Xmon qubits

12 4/17/18 Lykken | Perspectives on QIS | ASCAC Superconducting 3D quantum computers In 2011 the Yale group showed that there are great advantages to putting your Josephson Junction qubits inside superconducting microwave cavities

Good for: • Isolation • Control • Readout

Achieved record long quantum coherence times, as measured by T2, the dephasing time of H. Paik et al, PRL 107, 240501 (2011) superposition states

13 4/17/18 Lykken | Perspectives on QIS | ASCAC 3D qubits/cavities give the best quantum coherence

T2 of ~ a millisecond means you can think about quantum circuits with depth ~ 10,000

Can we do even better?

14 Lykken | Perspectives on QIS | ASCAC 4/17/18 The science of superconducting cavities At Fermilab we use superconducting cavities in the microwave/RF frequency band for particle accelerators

15 4/17/18 Lykken | Perspectives on QIS | ASCAC The science of superconducting cavities At Fermilab we make SRF cavities from niobium, and assemble them into cryomodules that in turn are assembled into linear accelerators

Alex Romanenko and Anna Grassellino lead the Fermilab SRF cavity program

Cryomodule built at Fermilab for the new LCLS-II free electron source at SLAC

16 4/17/18 Lykken | Perspectives on QIS | ASCAC The Q of superconducting cavities For an accelerator you want to achieve very high accelerating gradients (e.g. 30 megavolts/meter), and very high quality factor Q High Q means that the resonant cavities “ring” longer and thus need less microwave power pumped into them

Thanks to recent breakthroughs by Fermilab scientists, we now routinely achieve Q near or above 1011

If Galileo had rung a bell with this value of Q, it would still be ringing today

17 4/17/18 Lykken | Perspectives on QIS | ASCAC Niobium SRF cavities for quantum computers?

Challenges: • For accelerator cavities you want high gradients = as many photons as possible; for a quantum computer you want cavities containing a single microwave photon • Accelerators operate at temperatures around 2K, quantum computers around 20 milliKelvin

18 4/17/18 Lykken | Perspectives on QIS | ASCAC First try at T ~ 12 mK, down to ~1000 photons

Q ~ 2 x 109

100 times better than previous record

E ~ 1000 photons

Publication with full details in preparation

19 4/17/18 Lykken | Perspectives on QIS | ASCAC Why does Fermilab care about superconducting qubits? Fermilab is devoted to basic research in high energy (HEP)

20 4/17/18 Lykken | Perspectives on QIS | ASCAC HEP experiments use a lot of computing

21 Lykken | Perspectives on QIS | ASCAC 4/17/18 HEP applications of quantum computers

Long view: • Most applications will require thousands, if not millions, of error-corrected qubits, which won’t be available for ~20 years • However Fermilab is planning experiments that will be running 20 years from now, e.g. the DUNE experiment, and the CMS experiment at the LHC

What can we do now? • Identify pieces of problems that we care about that can be addressed with near-term quantum technologies

22 4/17/18 Lykken | Perspectives on QIS | ASCAC Applications of today’s quantum computers: learning

• Many particle physics experiments already using deep learning to better classify, e.g. neutrino-induced interactions in particle detectors • Some standard machine learning techniques, e.g. Boltzmann machines, involve estimating the ground state of a Hamiltonian that has many local minima

23 4/17/18 Lykken | Perspectives on QIS | ASCAC Applications of today’s quantum computers: with quantum annealers

• Annealing is a standard classical process for trying to avoid getting stuck in a local minimum • improves this by adding the possibility of quantum tunneling

• D-Wave makes a quantum computer with 2,000 superconducting qubits that functions as a quantum annealer • This is an analog quantum computer, not a gate-programmable computer

24 4/17/18 Lykken | Perspectives on QIS | ASCAC Quantum machine learning to identify Higgs • This has been applied to classifying CMS collisions events from the , to better identify events that produce a Higgs • The quantum annealer trains faster than a classical annealer • Is this useful in practice? Too soon to tell.

A. Mott et al, Nature 19 Oct 2017

Ned Allen and Adam Salamon (Lockheed-Martin), (Caltech)

25 4/17/18 Lykken | Perspectives on QIS | ASCAC Quantum simulation: from to bosons to gauge

26 4/17/18 Lykken | Perspectives on QIS | ASCAC Qubits for detection?

So far everything we know about dark matter is from its gravitational effects out in the cosmos

No one has detected dark matter particles in the laboratory

Gravitational lensing by dark matter Distribution of dark matter distorts galaxy images needed to explain the lensing

27 4/17/18 Lykken | Perspectives on QIS | ASCAC 27 Qubits for dark matter detection

There are many plausible theories for the identity of dark matter

• Superpartner particles: Wino, Bino, Higgsino, sneutrino, … • Axions • Kaluza-Klein particles from extra dimensions • Asymmetric dark matter • WIMPzillas (don’t ask…)

Dark matter streaming in from space may interact very weakly with ordinary matter

But the details depend on the mass and other properties of the dark matter particles

28 4/17/18 Lykken | Perspectives on QIS | ASCAC 28 Searching for “axion” dark matter particles

managed) A resonant cavity “axion” dark matter search - proceeds by tuning the radio frequency of the cavity and checking to see if you can hear the dark matter

Fermilab “radio broadcast” above the static noise

Simulated axion signal from ADMX

ADMX Experiment at U.Washington DOE Gen2 Dark Matter Project ( Project Gen2Matter DOE Dark

29 Lykken | Perspectives on QIS | ASCAC 4/17/18 Even at zero temperature, amplifiers for signal readout must add

Signal wave Amplification

to zoom in on details of shape

If no added noise, then one could simultaneously obtain arbitrarily precise information about both amplitude and phase, in violation of the Heisenberg

30 Lykken | Perspectives on QIS | ASCAC 4/17/18 Quantum-limited amplifiers (T0 K) suffer from zero- point readout noise This is called the Standard Quantum Limit (SQL)

(momentum)

Heisenberg Uncertainty Principle ∆푋 ∆푃 = ∆푁 ∆휃 = ½ ħ (quantum of phase space area)

(position)

Quantum noise = 1 photon of “zero point” noise per mode in the T=0 limit. (Caves, 1982)

31 Lykken | Perspectives on QIS | ASCAC 4/17/18 But wait: Quantum non-demolition (QND) single photon detectors can do much better than SQL amplifiers

Measure photon number and put all backreaction into the unobserved phase of the wave – which we don’t care about...

Demonstrated with Rydberg atoms, ( Nobel Prize 2012)

Implementation using superconducting qubits, D.Schuster et.al, 2007 Phase space area is still ½ħ but is squeezed in Proposed for axion search: radial (amplitude) Lamoreaux, Lehnert, et.al, direction. Phase of wave 2013, Zheng, Lehnert, et.al, is randomized. 2016

32 Lykken | Perspectives on QIS | ASCAC 4/17/18 Technology transfer from quantum computing to particle physics

Develop superconducting qubit-based single microwave photon detectors to enable a next generation dark matter Aaron Chou, David Schuster (UC), Daniel Bowring axion search.

New Fermilab test stand incorporates magnet into a Grad student Akash Dixit dilution refrigerator to simulate installing a prototype the extreme environment of a detector in a 10 mK test stand in the Schuster Lab. dark matter axion experiment.

33 4/17/18 Lykken | Perspectives on QIS | ASCAC Cold atom • It is straightforward to make an optical photon is a superposition of two states using beamsplitters • More recently the Stanford group does the same thing with cold atoms

Dickerson et al, PRL 111, 083001 (2013)

34 4/17/18 Lykken | Perspectives on QIS | ASCAC MAGIS-100 interferometry at large scales Sensor concept Atoms in free fall Atom Source 1 Probed using common laser

pulses 50 meters

Atom Quantum Source 2 superposition meters 100

Matter wave 50 meters Atom interference Source 3 pattern readout Quantum initiative Atom matter waves in superposition separated by up to 10 meters, durations up to 9 seconds Entangled atom source (spin squeezing) for increased sensitivity

Fundamental physics applications Time-dependent signals from ultra-light dark matter CAD model of detector in Fill the gravity wave frequency gap between LIGO and LISA 100-meter MINOS shaft at Fermilab

35 4/17/18 Lykken | Perspectives on QIS | ASCAC Particle physics and

• The Einstein-Podolsky-Rosen “Paradox” involves the strange properties of quantum entanglement

• In modern language this involves two entangled qubits, qA and qB

• qA is in a superposition of |0> and |1>, as is qB • But this “EPR pair” is 100% correlated; this can be represented as a , e.g.

36 4/17/18 Lykken | Perspectives on QIS | ASCAC Particle physics and quantum entanglement

• Now suppose that we separate the EPR pair

• When Alice measures qA, the state of qB is at that same moment predicted with 100% certainty • Einstein called this “spukhafte Fernwirkung” • Bob can verify this, but he must wait to get Alice’s prediction, since she cannot transmit her prediction faster than the

37 4/17/18 Lykken | Perspectives on QIS | ASCAC Quantum

EPR pairs can also be used for :

• Suppose Alice acquires an unknown single qubit photon state:

• If she measures it, it projects into one of the two computational basis states |0> or |1> • She cannot copy it, because of the no-cloning theorem

• Nevertheless if she and Bob already share the two members of an EPR photon pair, then Alice can teleport her unknown quantum state to Bob

38 4/17/18 Lykken | Perspectives on QIS | ASCAC Quantum teleportation protocol • Alice entangles the two photons in her possession • She measures her two photons in the computational basis, getting one of 4 possible results: |00>, |01>, |10>, |11> • Alice sends Bob the result of her message • Depending on Alice’s message, Bob performs a particular single qubit operation on his photon • Bob is now guaranteed to have the same unknown quantum state that Alice had

39 4/17/18 Lykken | Perspectives on QIS | ASCAC Fermilab quantum teleportation experiment (FQNET) • Uses laser, attenuator, and arbitrary waveform generator to make time-binned optical photonic qubits • Teleport the qubits over 40 km of commercial fiber • Measure with SNSPDs = Superconducting Nanowire Single Photon Detectors

40 4/17/18 Lykken | Perspectives on QIS | ASCAC Fermilab quantum teleportation experiment (FQNET)

Through the INQNET research program and Caltech, AT&T supports the pilot quantum teleportation experiment at Fermilab The Chicago Quantum Exchange is making plans with INQNET for establishing a testbed connecting Fermilab to Argonne.

John Donovan, CEO AT&T Communications

41 4/17/18 Lykken | Perspectives on QIS | ASCAC Quantum teleportation through a • It has been known for decades that the extended Schwarzschild describes a pair of black holes connected via a wormhole, known as the Einstein-Rosen (ER) bridge. • This wormhole is non-traversable, in the sense that anyone jumping into one , and attempting to get to the horizon of the second black hole, instead ends up at the singularity. • One can make a similar pair of black holes in Anti-de-Sitter space; in this case the AdS/CFT correspondence gives a powerful relation between the bulk gravitational physics and the physics of quantum entanglement

Penrose diagram of a pair of black holes connected by an ER bridge

42 4/17/18 Lykken | Perspectives on QIS | ASCAC Quantum teleportation through a wormhole • In 2016 Gao, Jafferis and Wall showed that the AdS version contains a traversable wormhole when perturbed in a simple way. • Maldacena, Stanford and Yang have shown the required perturbation can be mapped into the standard operations of quantum teleportation. • Thus, in this particular kind of semi-classical system, quantum teleportation has an equivalent AdS/CFT description as a qubit physically moving through a wormhole.

Maldacena and Susskind have speculated that EPR = ER in general, i.e. that quantum teleportation in Bob Alice general is some kind of “quantum Tom wormhole”

P. Gao, D. Jafferis, A. Wall, “Traversable via a double trace deformation”, arXiv:1608.05687. J. Maldacena and L. Susskind, “Cool horizons for entangled black holes”, arXiv:1306.0533. J. Maldacena, D. Stanford, Z. Yang, “Diving into traversable wormholes”, arXiv:1704.05333. L. Susskind and Y. Zhao, “Teleportation ”, arXiv:1707.04354.

43 4/17/18 Lykken | Perspectives on QIS | ASCAC 44 4/17/18 Lykken | Perspectives on QIS | ASCAC 45 4/17/18 Lykken | Perspectives on QIS | ASCAC 46 4/17/18 Lykken | Perspectives on QIS | ASCAC How should the labs engage in the DOE SC initiative?

Fermilab is engaging in ways appropriate to our role as the main HEP lab:

• Focus on the science • Find partners who already have leading quantum science expertise • Exploit existing Fermilab expertise and infrastructure • Leverage HEP resources with other funding streams • Keep Fermilab activity aligned to HEP needs • We are a gateway for the HEP community, so become an HEP hub for quantum science • Produce high impact science results in the near term, while also building capacity for HEP needs in the long term

47 4/17/18 Lykken | Perspectives on QIS | ASCAC Fermilab quantum collaborations Collaborations are for non-proprietary basic research in quantum science. Strategy is to engage with major U.S.-based companies, other labs, and university groups already expert in QIS.

48 4/17/18 Lykken | Perspectives on QIS | ASCAC Conclusions • is developing fast • The community of scientists engaged in advancing or applying these technologies is also growing fast and becoming broader • But patience and a long view will be required

49 4/17/18 Lykken | Perspectives on QIS | ASCAC